Field of the Invention
The present invention relates to a density correction technique in an image forming apparatus.
Description of the Related Art
Conventionally, a so-called electrophotographic image forming apparatus is known, such as a laser beam printer that forms an electrostatic latent image by irradiating a photoconductor drum with laser light and a copy machine. In such an image forming apparatus, gamma correction processing to correct so-called gamma characteristics (input/output characteristics) is performed in order to match a target density with a density to be printed actually. In general, in the gamma correction processing, gamma correction characteristics (correction table) are used to correct the gamma characteristics.
In such an image forming apparatus, there is a case where the actual gamma characteristics deviate from the original gamma characteristics due to the deterioration of a photoconductor drum with the passage of time, the change in the environment, such as temperature and humidity, etc. Because of this, it is common to perform calibration for the gamma correction processing periodically or at timing at which the output error exceeds a threshold value.
In an electrophotographic image forming apparatus, unevenness in density sometimes occurs within a printing surface even in the case where an image with uniform gradation is formed due to the manufacturing error and the assembling error of constituent parts and supports. Such unevenness in density will cause a reduction in the accuracy of calibration for the gamma correction processing. Japanese Patent Laid-Open No. 2007-057954 has disclosed a technique to store corresponding gamma correction characteristics for each of a plurality of positions in the main scanning direction in order to improve the unevenness in density itself.